Is your school a FFELP school or a Direct Loans school? If it is a FFELP school you can use any lender for the GradPlus you want. You may have to make special arrangements to get your loans disbursed and really stay on top of things to use a lender that isn't on your school's preferred list but it is totally possible.
In years past, many lenders offered the option of getting a cosigner or endorser if you couldn't qualify on your own. Some companies may have ditched that due to the financial crisis. I would just start calling lenders and ask them. Make sure you ask if they will lend a borrower attending your school before you do anything else though.
Here is a list of lenders I found online. It's probably not comprehensive but should get you started:http://www.edfund.org/pdfs/FFEL_Program_Lenders_Schools.pdf
Make sure your cosigner or endorser understand that they will have to pay back the loan if you can't and make sure they have good credit. If they don't have good credit, you will not get a loan. Also, keep in mind this person will have to re-authorize and sign off each year for your loans. If they decide not to during 2L and 3L, you could be screwed.
In general, I would try to clear up any credit issues before going to law school. Even if you can get a grad plus loan somehow for all 3 years, you may have trouble with the character and fitness portion of your state's bar exam.